MPI_Cart_map and MPI_Graph_map
can be used to implement all other topology functions. In general they will
not be called by the user directly, unless he or she is creating additional
virtual topology capability other than that provided by MPI.

MPI_Cart_map
computes an "optimal" placement for the calling process on the physical
machine. A possible implementation of this function is to always return
the rank of the calling process, that is, not to perform any reordering.

Almost all MPI routines return an error value; C routines as the
value of the function and Fortran routines in the last argument. C++ functions
do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS,
then on error the C++ exception mechanism will be used to throw an MPI:Exception
object.

Before the error value is returned, the current MPI error handler
is called. By default, this error handler aborts the MPI job, except for
I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler;
the predefined error handler MPI_ERRORS_RETURN may be used to cause error
values to be returned. Note that MPI does not guarantee that an MPI program
can continue past an error.